1. Field of the Invention
The present invention concerns a method and device allowing the protected detection of the pollution of water.
2. Description of the Prior Art
Generally speaking, it is known that so as to detect pollution thresholds, the fish, usually trout, are placed in a container inside which the water is to be controlled circulates, and parameters representing the behaviour of the fish are detected. An alarm signal is transmitted when the detected parameters show an abnormal behaviour of the fish, said alarm likely to be generated by a pollution threshold being exceeded.
Thus, in particular in the French patent No. 98 01387 filed in the name of the applicant, a method is being proposed consisting of:                generating in the water of the container a series of ultrasonic wave trains spaced from one another by periods of silence,        detecting during the periods of silence the echoes generated by each of said trains,        carrying out the temporal analysis of said echoes and memorising the results of theses analyses,        comparing the results of the time analyses of the echoes resulting from each wave train with reference values and/or with the results of the time analysis of the echoes resulting from the preceding wave train,        determining the degree of pollution of the water according to the results of said comparison.        
This method is based on the fact that the form and the layout of the echoes (variation in the process of time of analysis of the signal) depend on the position of the various obstacles and that consequently, by comparing the form of various echoes detected during two successive silent periods, it is possible to determine a modification of the movement of the fish. In the case of no movement occurring, the echoes shall always be identical and there shall be no difference between two successive echoes. On the other hand, when the fish move, the difference between two echoes is proportional to the amount of movement. An alarm could be triggered below or above an adjustable threshold between two echoes.
This solution proves to have a significant drawback owing to the fact that like most animals, trout are subject to going to sleep. Consequently, during periods of sleep, usually at night, the behaviour of the trout is closely related to abnormal behaviour giving rise to the transmission of the alarm signal. As a result, false signals are triggered, thus rendering this method unusable during the periods when the trout are likely to sleep.